Physics · Thermal Properties Of Matter · NEET
When a solid melts or a liquid boils, the molecules are held together by intermolecular bonds. The heat you supply is used to break these bonds and increase the potential energy of the molecules, not their kinetic energy. Temperature is a measure of the average kinetic energy. Since kinetic energy does not change during the change of state, the temperature reading stays constant. Once all the substance has changed state, extra heat then starts raising the temperature again.
Yes. Heat keeps flowing in the whole time, but it is stored as latent heat, not as a temperature rise. The formula is Q = m L, where L is the latent heat and m is the mass. For example, melting 1 kg of ice at 0 degree C needs Q = 1 x 3.33 x 10^5 = 3.33 x 10^5 J, all absorbed with zero temperature change. This is why ice keeps a drink cold for a long time: it soaks up heat while staying at 0 degree C.
Melting point is the fixed temperature at which the solid and liquid states of a substance stay in thermal equilibrium (both exist together), for example 0 degree C for ice. Boiling point is the fixed temperature at which the liquid and vapour states stay in thermal equilibrium, for example 100 degree C for water. Both are characteristic of the substance and both depend on pressure. At melting the latent heat of fusion is absorbed; at boiling the (larger) latent heat of vaporisation is absorbed.
Boiling point rises when pressure increases and falls when pressure decreases. In a pressure cooker the pressure is high, so water boils above 100 degree C and food cooks faster. On high hills the atmospheric pressure is low, so water boils below 100 degree C. This lower-temperature boiling water cannot cook food properly, which is why cooking is difficult at high altitudes.
Steam and boiling water are at the same temperature, but steam carries extra hidden energy: the latent heat of vaporisation. When 1 g of steam touches your skin and condenses to water, it releases about 2256 J (the latent heat of steam) before it even starts to cool. That is far more than boiling water gives, so steam causes a much more severe burn. This shows latent heat is real stored energy.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It depends on the direction. Melting and boiling (solid to liquid, liquid to vapour) absorb heat. Freezing and condensation (liquid to solid, vapour to liquid) release the same amount of heat. The magnitude of latent heat is the same in both directions for a given change.
For water the latent heat of fusion is about 3.33 x 10^5 J/kg (334 J/g) and the latent heat of vaporisation is about 22.6 x 10^5 J/kg (2256 J/g). Vaporisation needs much more heat because the molecules must be completely separated into vapour.
Sublimation is a change directly from solid to vapour without passing through the liquid state. Dry ice (solid carbon dioxide) and iodine sublime. During sublimation the solid and vapour coexist in thermal equilibrium, and heat is absorbed at a fixed temperature just like other changes of state.
The length of a plateau depends on how much heat is needed, and heat is Q = m L. Because the latent heat of vaporisation is much larger than the latent heat of fusion, boiling needs far more heat for the same mass. With a constant heat source, more heat means more time, so the boiling plateau is much longer than the melting plateau.
